1. Filed of the Invention
The present invention relates to a printed board and an electronic apparatus.
2. Description of the Related Art
Generally, electronic parts or the like constituting an electronic apparatus are mounted on a printed board. The printed board has an electrically insulating board which is made of a glass epoxy material or the like and which has one surface on which electronic parts will be mounted, and the other surface on which a conductor layer made of a conductor pattern of a circuit is formed by means of etching a sheet of copper foil or the like. The printed board further has through-holes which are formed in the electrically insulating board so that an inner wall of each through-hole is covered with a conductor by which conductors provided on the front and rear surfaces of the electrically insulating board are connected to each other so that the electronic parts can be electrically connected to the conductor layer.
Lands for electrical connection are provided on circumferences of the through-holes respectively. A resist layer of a solder resist film or the like is provided so that the conductor layer on the front surface is covered with the resist layer except that the lands on the circumferences of the through-holes are exposed. Paste-like ink or the like is applied onto a front surface of the resist layer by silk-screen printing so that symbols or marks are printed on the front surface of the resist layer.
For example, as disclosed in JP-A-8-125288 (Page 1 and FIG. 1) which is a conventional technique, protection marks are silk-screen printed on a front surface of a solder resist film for covering an extended portion of a conductor pattern extending to the neighborhood of a circumferential portion of an electrically insulating board so that the protection marks are arranged along the circumferential portion. The protection marks abut against a support portion of a manufacturing apparatus for mounting electronic parts or the like, so that the solder resist film and the conductor pattern are prevented from being damaged.
For example, as disclosed in JP-A-6-302987 (Page 1 and FIG. 1) which is a conventional technique, an electrically conductive shield plate is bonded to a front surface of a printed board to be mounted in an acoustic apparatus, a communication apparatus or the like in order to prevent interference with unnecessary external radio wave (noise). When the shield plate is to be bonded, each through-hole portion is covered with a nonconductive symbolic layer formed by silk-screen printing so that an electrically conductive layer on an inner wall of each through-hole is prevented from coming into contact with the shield plate because of the shield plate's bending at the through-hole portion.
As described above, the through-holes and the lands are exposed on the front surface of the background-art printed board because the resist layer is formed so as to avoid the lands on the circumferences of the through-holes. For this reason, it cannot be said that the background-art printed board has satisfactory resistance to a high-temperature high-humidity environment because water may go into the printed board through the exposed through-holes and lands. There is hence a possibility that the front surface of the printed board may be corroded due to oxidization, or the insulation resistance of the printed board may be lowered due to a migration phenomenon. Therefore, a printed board more greatly improved in resistance to the high-temperature high-humidity environment has been demanded for use in an on-vehicle electronic apparatus or the like.
To satisfy the demand, it may be conceived that a protective coating agent is applied to the front surface of the printed board to eliminate the exposed portions to thereby improve resistance to the high-temperature high-humidity environment. Extra cost and time are however spent inevitably because the protective coating agent and the coating process for the protective coating agent are required.
Incidentally, in the conventional technique described in JP-A-6-302987, the shield plate and the process for bonding the shield plate are required. Accordingly, extra cost and time are spent even on an electronic apparatus that can be provided without special attention paid to interference with unnecessary radio wave (noise).